Browsing by Author "Govorov, A. O."

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Open Access
Excitonics of hybrid nanostructures arranged with mixed dimensionality
(American Physical Society, 2012) Hernandez-Martinez, Pedro L.; Govorov, A. O.; Demir, Hilmi Volkan
We present a complete study of the F\"{o}rster-type nonradiative energy transfer in hybrid nanostructures composed of nanoparticles, nanowires and quantum wells, and investigate the effects of quantum confinement in different dimensions. We systematically consider all possible combinations in terms of dimensionality for exciton-exciton interactions in these hybrid architectures, and analyze the resulting energy transfer rates for item-to-item excitonic coupling as a function of dimensionality. We derive a full set of analytical expressions and show that the exciton transfer strongly depends on the dimensionality and geometry of the hybrid system. Arrangements of such nanostructures with mixed dimensionality ranging from the low dimensionality to the high offer important high-efficiency applications in photovoltaics [1,2], while in the reciprocal case (from the high dimensionality to the low) in light generation [3] and LEDs [4]. [1] J. Sambur, et al., Science 330, 63 (2010). [2] M. D. Kelzenberg, et al., Nature Materials 9, 239--244 (2010). [3] R. Yan, et al., Nature Photonics 3, 569-576 (2009). [4] H.V. Demir, et al., Nano Today (2011) doi:10.1016/j.nantod.2011.10.006
Open Access
Excitonics of semiconductor quantum dots and wires for lighting and displays
(Wiley-VCH Verlag, 2013) Guzelturk, B.; Martinez, P. L. H.; Zhang, Q.; Xiong, Q.; Sun, H.; Sun, X. W.; Govorov, A. O.; Demir, Hilmi Volkan
In the past two decades, semiconductor quantum dots and wires have developed into new, promising classes of materials for next-generation lighting and display systems due to their superior optical properties. In particular, exciton-exciton interactions through nonradiative energy transfer in hybrid systems of these quantum-confined structures have enabled exciting possibilities in light generation. This review focuses on the excitonics of such quantum dot and wire emitters, particularly transfer of the excitons in the complex media of the quantum dots and wires. Mastering excitonic interactions in low-dimensional systems is essential for the development of better light sources, e.g., high-efficiency, high-quality white-light generation; wide-range color tuning; and high-purity color generation. In addition, introducing plasmon coupling provides the ability to amplify emission in specially designed exciton-plasmon nanostructures and also to exceed the Forster limit in excitonic interactions. In this respect, new routes to control excitonic pathways are reviewed in this paper. The review further discusses research opportunities and challenges in the quantum dot and wire excitonics with a future outlook.
Open Access
Förster-type nonradiative energy transfer for assemblies of arrayed nanostructures: confinement dimension vs stacking dimension
(American Chemical Society, 2014-02-11) Hernandez-Martinez, P. L.; Govorov, A. O.; Demir, Hilmi Volkan
Forster-type nonradiative energy transfer (NRET) provides us with the ability to transfer excitation energy between proximal nanostructures with high efficiency under certain conditions. Nevertheless, the well-known Forster theory was developed for the case of a single donor (e.g., a molecule, a dye) together with single acceptor. There is no complete understanding for the cases when the donors and the acceptors are assembled in nanostructure arrays, though there are special cases previously studied. Thus, a comprehensive theory that models Forster-type NRET for assembled nanostructure arrays is required. Here, we report a theoretical framework of generalized theory for the Forster-type NRET with mixed dimensionality in arrays. These include combinations of arrayed nanostructures made of nanoparticles (NPs) and nanowires (NWs) assemblies in one-dimension (1D), two-dimension (2D), and three-dimension (3D) completing the framework for the transfer rates in all possible combinations of different confinement geometries and assembly architectures, we obtain a unified picture of NRET in assembled nanostructures arrays. We find that the generic NRET distance dependence is modified by arraying the nanostructures. For an acceptor NP the rate distance dependence changes from gamma alpha d(-6) to gamma alpha d(-5) when they are arranged in a ID stack, and to gamma alpha d(-4) when in a 2D array, and to gamma alpha d(-3) when in a 3D array. Likewise, an acceptor NW changes its distance dependence from gamma alpha d(-5) to gamma alpha d(-4) when they are arranged in a 1D array and to gamma alpha d(-3) when in a 2D array. These finding shows that the numbers of dimensions across which nanostructures are stacked is equally critical as the confinement dimension of the nanostructure in determining the NRET kinetics.
Open Access
Generalized theory of förster-type nonradiative energy transfer in nanostructures with mixed dimensionality
(American Chemical Society, 2013-04-16) Hernandez-Martinez, P. L.; Govorov, A. O.; Demir, Hilmi Volkan
Forster-type nonradiative energy transfer (NRET) is widely used, especially utilizing nanostructures in different combinations and configurations. However, the existing well-accepted Forster theory is only for the case of a single particle serving as a donor together with another particle serving as an acceptor. There are also other special cases previously studied; however, there is no complete picture and unified understanding. Therefore, there is a strong need for a complete theory that models Forster-type NRET for the cases of mixed dimensionality including all combinations and configurations. We report a generalized theory for the Forster-type, NRET, which includes the derivation of the effective dielectric function due to the donor in different confinement geometries and the derivation of transfer rates distance dependencies due to the acceptor in different confinement geometries, resulting in a complete picture and understanding of the mixed dimensionality.
Open Access
Phonon-assisted exciton transfer into silicon using nanoemitters: the role of phonons and temperature effects in förster resonance energy transfer
(American Chemical Society, 2013) Yeltik A.; Guzelturk, B.; Hernandez-Martinez, P. L.; Govorov, A. O.; Demir, Hilmi Volkan
We study phonon-assisted Forster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a d(-3) distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters.
Open Access
Photogeneration of hot plasmonic electrons with metal nanocrystals: quantum description and potential applications
(Elsevier Ltd, 2014-02) Govorov, A. O.; Zhang, H.; Demir, Hilmi Volkan; Gun’ko, Y. K.
he paper reviews physical concepts related to the collective dynamics of plasmon excitations in metal nanocrystals with a focus on the photogeneration of energetic carriers. Using quantum linear response theory, we analyze the wave function of a plasmon in nanostructures of different sizes. Energetic carriers are efficiently generated in small nanocrystals due to the non-conservation of momentum of electrons in a confined nanoscale system. On the other hand, large nanocrystals and nanostructures, when driven by light, produce a relatively small number of carriers with large excitation energies. Another important factor is the polarization of the exciting light. Most efficient generation and injection of high-energy carriers can be realized when the optically induced electric current is along the smallest dimension of a nanostructure and also normal to its walls and, for efficient injection, the current should be normal to the collecting barrier. Other important properties and limitations: (1) intra-band transitions are preferable for generation of energetic electrons and dominate the absorption for relatively long wavelengths (approximately >600 nm), (2) inter-band transitions efficiently generate energetic holes and (3) the carrier-generation and absorption spectra can be significantly different. The described physical properties of metal nanocrystals are essential for a variety of potential applications utilizing hot plasmonic electrons including optoelectronic signal processing, photodetection, photocatalysis and solar-energy harvesting. © 2014 Elsevier Ltd.
Open Access
Plasmonic metamaterials and nanocomposites with the narrow transparency window effect in broad extinction spectra
(American Chemical Society, 2014) Zhang, H.; Demir, Hilmi Volkan; Govorov, A. O.
We propose and describe plasmonic nanomaterials with unique optical properties. These nanostructured materials strongly attenuate light across a broad wavelength interval ranged from 400 nm to S pm but exhibit a narrow transparency window centered at a given wavelength. The main elements used in our systems are nanorods and nanocrosses of variable sizes. The nanomaterial can be designed as a solution, nanocomposite film or metastructure. The principle of the formation of the transparency window in the broad extinction spectrum is based on the narrow lines of longitudinal plasmons of single nanorods and nanorod complexes. To realize the spectrum with a transmission window, we design a nanocomposite material as a mixture of nanorods of different sizes. Simultaneously, we exclude nanorods of certain lengths from the nanorod ensemble. The width of the plasmonic transparency window is determined by the intrinsic and radiative broadenings of the nanocrystal plasmons. Nanocrystals can be randomly dispersed in a solution or arranged in metastructures. We show that interactions between nanocrystals in a dense ensemble can destroy the window effect and, simultaneously, we design the metastructure geometries with weak destructive interactions. We also describe the effect of narrowing of the transparency window with increasing the concentration of nanocrystals. Two well-established technologies can be used to fabricate such nano- and metamaterials, the colloidal synthesis, and lithography. Nanocomposites proposed here can be used as optical materials and smart coatings for shielding of electromagnetic radiation in a wide spectral interval with a simultaneous possibility of communication using a narrow transparency window.
Open Access
Simple and complex metafluids and metastructures with sharp spectral features in a broad extinction spectrum: particle-particle interactions and testing the limits of the Beer-Lambert law
(American Chemical Society, 2017) Besteiro, L. V.; Gungor K.; Demir, Hilmi Volkan; Govorov, A. O.
Metallic nanocrystals (NCs) are useful instruments for light manipulation around the visible spectrum. As their plasmonic resonances depend heavily on the NC geometry, modern fabrication techniques afford a great degree of control over their optical responses. We take advantage of this fact to create optical filters in the visible-near IR. Our systems show an extinction spectrum that covers a wide range of wavelengths (UV to mid-IR) while featuring a narrow transparency band around a wavelength of choice. We achieve this by carefully selecting the geometries of a collection of NCs with narrow resonances that cover densely the spectrum from the UV to the mid-IR except for the frequencies targeted for transmission. This fundamental design can be executed in different kinds of systems, including a solution of colloidal metal NCs (metafluids), a structured planar metasurface, or a combination of both. Along with the theory, we report experimental results, showing metasurface realizations of the system, and we discuss the strengths and weaknesses of these different approaches, paying particular attention to particle-particle interaction and to what extent it hinders the intended objective by shifting and modifying the profile of the planned resonances through the hybridization of their plasmonic modes. We found that the Beer-Lambert law is very robust overall and is violated only upon aggregation or in configurations with nearly touching NCs. This striking property favors the creation of metafluids with a narrow transparency window, which are investigated here.
Open Access
Study of exciton transfer in dense quantum dot nanocomposites
(Royal Society of Chemistry, 2014) Guzelturk, B.; Hernandez-Martinez, P. L.; Sharma, V. K.; Coskun, Y.; Ibrahimova, V.; Tuncel, D.; Govorov, A. O.; Sun, X. W.; Xiong, Q.; Demir, Hilmi Volkan
Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ∼70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies.